Power control in RF transmitter systems is well known in the art and is commonly used to control transmitter output power depending on the proximity of the transmitter to a receiver in a radio or cellular telephone network. The power output may be controlled using radio signal strength indication (RSSI) measurement or otherwise so the receiving radio is not overloaded and to reduce power consumption in the transmitting communications device.
Controlling drive power of an RF amplifier is often essential in minimizing the amount of current drain in portable devices. U.S. Pat. No. 6,681,101 and U.S. Pat. No. 6,684,064, which are herein incorporated by reference, describe methods of controlling RF drive power. The '064 patent describes a dynamic bias for an RF power amplifier where operating bias of the RF transmitter is dynamically adjusted in response to a power control signal. The '101 patent describes an RF transmitter with an extended efficient power control arrangement where the transmitter includes stage switching, bias adjustment and drain supply modulation to provide fine and coarse power control and power envelope fluctuations. Although these systems operate to allow a reduction in power in an RF amplifier stage, they do not operate to conserve current drain or to trade off distortion for current. These prior art systems do not use a multistage approach where current is more efficiently used by controlling output power in both the low current driver stage as well as the high current amplifier stage while still maintaining overall linearity and high efficiency. When these types of transmitter power controls are used with portable devices, they do not function to reduce current drain since their design is solely for the purpose of varying the power output of the RF amplifier. These prior art systems are fixed at one current level for meeting specific design requirements such as linearity at power regardless of output power needed. Consequently, these designs can be highly inefficient and wasteful leading to high current drain and a shorter than necessary battery life. This can be very problematic in portable RF devices where battery life is of critical concern.
Accordingly, the need exists to provide a system and method for controlling RF power output in an RF amplifier while still operating to have an efficient current drain while maintaining linearity of an RF power amplifier.